Figuring out a program communication

I have an embedded gnu/linux on an arm device running a program and I can telnet it. I want to know how the program sends commands to the device so I can make my own program to send those commands but when I want it to. I'm pretty sure it writes to something in /dev.

How do I know which file in /dev (I know its not really files) a program is writing to and what?

For reference its on armv5tejl chip with 2.6.27.47 kernel. I also have its tool-chain开发者_JAVA百科 so I can compile programs to it.

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Using lsof (list open files), you can see which files each process has open. You should find the device node your process uses there. Alternatively, you can find out the PID (process ID) of your program using ps aux, then look at the open file descriptors of the process at /proc/$pid/fd, where $pid is the PID of your program.

To find out what the program is writing, probably the easiest thing is to use strace to trace all system calls the program does. (With strace, you can also find out which file the program opens.) You could also replace the file the program writes to with an empty file, if possible, or, if necessary, with a dummy kernel driver, which records everything it receives.